This invention is related, in general, to demodulation circuits and, more specifically, to Frequency Modulation (FM) and Frequency-Shift Keying (FSK) demodulation circuits.
Many portable communication products utilize circuits that can perform well in a low power environment. The communication products have a front-end receiver designed for cellular or personal communications service (PCS) that converts a Radio Frequency to Intermediate Frequency (RF-to-IF). The information encoded in the signal received in a two-way communication link by the receiver in these handheld devices is recovered using demodulator circuits. The receiver typically integrates Low Noise Amplifiers (LNAs), quadrature phase shift networks and mixers for down converting the received signal from RF to IF and a demodulator for producing the recovered output signal. The recovered signal is proportional to the deviation of the frequency of the received signal relative to the center frequency of an oscillator found in the demodulator circuit. The receiver may operate in the 800-900 MHz cellular band and the 1800-1900 MHz PCS band, as well as other frequency bands.
A problem associated with most prior art FM demodulator circuits is the variation in output amplitude and bandwidth with process and temperature. One type of demodulator, i.e., the quadrature demodulator, generates an output signal having an amplitude that varies with the gain and bandwidth that varies with a phase shift network and with the input signal level. Another type of demodulator, i.e., the phase-locked loop demodulator, generates an output signal having an amplitude that varies with the input signal level, as well as capacitor values and resistor values that determine the gain of the VCO. Prior art injection-locked demodulators also generate an output signal that varies in amplitude depending upon capacitor and resistor values. The bandwidth of these types of prior art demodulators is not constant, but varies either with component values or the loop filter, or both.
Hence, there exists a need for a demodulation circuit that produces a recovered output signal that is less sensitive to amplitude variations in the received input signal and has a well-defined locking range and bandwidth.